Zinc sulfide and (ZnCd)S activated with either Cu, Ag or Au and coactivated with either Cl, Al, Ga or In are well known as efficient photo and cathodoluminescent phosphors. Zinc and zinc-cadmium chalcogenide phosphors activated by the rare earth elements are also known. Luminescence from ZnS:Tm was described by S. Rothschild, Proc. Int. Conf. on Sol. State Phys. and Telecomm., 4, Pt. 2, 705 (1958), who found an emission manifold centered in the blue spectral region at 478 nm. Ibuki and Langer, J. Chem. Phys., 40, 796 (1964), also studied the fluorescence of ZnS:Tm and concluded that the Tm.sup.3+ ion occupied a lattice site with Td symmetry. Anderson, S. Razi and D.J. Walsh, ibid, 43, 1153 (1965); W. W. Anderson, ibid, 44, 3282 (1966) investigated the fluorescence of a several rare earth ions in zinc and cadmium sulfide matrices and noted that the trivalent rare earth can associate with a variety of acceptor defects, and it thereby occupies a number of non-equivalent symmetry sites. Larach, Proc. Int. Conf. on Lumin., Budapest, 1966, 1549 ( 1968) extended the work of Rothschild and reported two additional emissions from ZnS:Tm, one centered at 645 nm and another at 775-800 nm, with about 20% of the total energy in the near infrared manifold. In a subsequent patent, Larach and Yocom (U.S. Pat. No. 3,459,969) of RCA Corp. detailed the preparation of luminescent materials consisting of the sulfide or sulfo-selenides of zinc and cadmium, containing between 0.001 to 1.0 mole percent of a rare earth element selected from the group consisting of Pr, Nd, Sm, Tb, Dy, Ho, Er, Tm and between 0.0001 and 1.0 mole percent of an alkali metal selected from a group consisting of Li, Na, K, Rb, Cs. These workers theorized that the function of the monovalent alkali metal was to provide charge compensation for the trivalent rare earth ion when it is substituted for divalent zinc or cadmium in the sulfo-selenide lattice. Although the exact function of the alkali metal was not understood it was found to be essential to the preparation of improved phosphors. Moreover, the function of the incorporated alkali was found to be different from that of the monovalent Ag, Cu and Au used in the prior art phosphors. In fact the earlier workers teach that Ag and Cu does not produce the advantages effect of the alkali metal. With Ag or Cu present it was found that a broad band emission typical of these ions accompanied the narrow emission bands of the rare earth in the cathodoluminescence spectra. Shrader, Larach and Yocom, J. Appl. Phys., 12, (1971) also measured the cathodoluminescence efficiency of ZnS:Tm and found it to be the highest reported for a rare-earth activated phosphor. These workers emphasized the unusual power distribution among the three emission manifolds that concentrates the major portion of the ZnS:Tm luminosity in a single narrow blue band.
Finally, in more recent work, Charreire et al., Mat. Res. Bull., 15, 657 (1980) conducted a detailed spectral study of the blue emitting Tm.sup.3+ manifold and its relationship to the ZnS crystal structure. These workers concluded that the Td symmetry site occupied by the Tm.sub.3+ ion is similar in all of the various ZnS structure types (Cubic, hexagonal and faulted) and is independent of charge compensation.